1. Technical Field of the Invention
The present invention relates generally to shear stress determination and more particularly to determining shear stress via changes in light wavelengths reflected via a monomer-polymer liquid crystal system.
2. Discussion of the Related Art
Cholesteric liquid crystals, monomers as well as polymers, demonstrate the phenomenon of selective reflection where incident white light is reflected in such a way that its wavelength is governed by the instantaneous pitch of the helix structure of these phases. The helical structure can be modulated by thermodynamic as well as mechanical perturbations, e.g., by changing the temperature or by applying an external stress field. The helix pitch may, however, be compensated for temperature effects, thereby leaving it sensitive only to external shear stress. Because of the high viscosity of liquid crystal polymers (LCPs), the shear stress fields required for helix structure modulation in cholesteric LCPs are very high and therefore LCPs are not good candidates for low shear indicators. Shear sensitive, but temperature insensitive, monomer cholesteric liquid crystals are low viscosity phases and their structure can be modulated by relatively low stress fields. As a result, monomer cholesteric liquid crystals have found application in flow visualization and surface temperature measurement in aerodynamic testing as well as in subsonic and supersonic wind tunnel experiments. Many of these experiments have provided significant qualitative information about the flow field. These experiments have not been so successful in providing quantitative data on the flow parameters, especially when the liquid crystal thin films on model surfaces are exposed directly to the wind flow. The lower viscosity of the monomer liquid crystals combined with the poor wettability of the conventional model surfaces results in the thinning and ultimately washing out of the liquid crystal when exposed to wind flow. As a consequence, the selective reflection characteristics of the model surface on which a monomer cholesteric thin film has been applied become time dependent for a given rate of wind flow.